That was an interesting article. Thanks for posting it.Here is more on those Heinkels which operated from Calshot. My first job was next door to where they first landed in Hythe Eye on Millig: Inside cockpit of German WW2 Heinkel bomber based at Helensburgh
It described the rear cockpit modification that OllieReeder has already mentioned - an electrically operating sliding canopy.
The Arado 196 that the FAA briefly owned was being flown to Helensburgh when it crashed.